Least what? Anyway, heat energy is in a way the least useful form of energy, because you can normally convert any other energy to heat energy, but if you have heat energy, you can no longer convert all of it back to other forms of energy.
Ambient heat
Heat and electricity at the very least.
The least usable form of energy is thermal energy, such as low-temperature heat, because it is difficult to harness efficiently and convert into other forms of energy. Thermal energy is often dispersed and not easily concentrated or converted into useful work.
That's because it is not possible to convert the heat to other forms of energy - at least, not all of it.
The top level, which consists of tertiary consumers, contains the least energy in the energy pyramid because energy is lost as heat as it moves up the trophic levels.
Water has a high specific heat capacity, meaning it can absorb a large amount of heat energy while experiencing only a small temperature change. Therefore, one kilogram of water would absorb the most heat energy with the least temperature change compared to other substances.
Ultimately - the Sun, or at least the radiated light and heat energy given off by it
No, at least, not all of it. If there is a difference in temperature between two objects, part of the heat can be converted into other forms of energy.
No, water is actually quite efficient at absorbing and retaining heat from the sun's energy due to its high specific heat capacity. This means it can absorb a lot of heat without a large increase in temperature compared to other substances.
Sand needs least energy to increase the temperature as its specific heat is very low compared to that of water. Water needs more energy to increase its temperature as its specific heat capacity is higher.
Geothermal energy is the renewable resource that is least dependent on the sun. It harnesses heat from the Earth's core, providing a continuous and reliable source of energy regardless of sunlight or weather conditions.
Thermal energy is the form of energy least available to accomplish cellular work, as it represents random kinetic energy associated with the movement of molecules that is not easily harnessed for specific cellular processes.